Brown the mince in a large pan over a medium high heat, breaking it up with a spoon, until there is no pink left. Add the onion and cook for 8 minutes until soft.
Crumble in the stock cube, add the diced potatoes, the Worcestershire sauce and 400ml water. Cover and simmer for 20-25 minutes until the potatoes are tender and the gravy has thickened. Season well, then set aside to cool slightly.
Meanwhile, rub the butter or margarine into the flour with a pinch of salt until it resembles breadcrumbs, then add just enough cold water to bring it into a soft dough. Chill for 20 minutes.
Heat the oven to 200C/180C fan/Gas 6. Roll out two thirds of the pastry and use it to line a 20cm pie tin. Spoon in the cooled meat and potato filling.
Roll out the remaining pastry, lay it over the top, trim the excess, seal and crimp the edges, and cut a steam hole in the middle.
Bake for 35-40 minutes until the pastry is deep golden. Rest for 10 minutes, then serve with mushy peas or extra gravy.